View Full Version : SMD LED what do you think the current is

Protolisk

12-10-2008, 07:59

I hace some SMD LEDs except it doesn't say the current so I dont know what resistor to use.

2.2V

5mcd

120°

Amber colour

They had some mix up with the colours so it might be yellow which means that it has

2.1V

6mcd

Just want to know around how much the forward current is? or a reccomended resistor.

Dr_Acula

12-10-2008, 08:20

Try 1k. A safe sort of value. You could go down to 220 ohms if you like.

Which resistor value you use, depending what is your's voltage.

U=Voltage

I=Current

R=Resistor value (Ohms)

R=U/I

I=U/R

This calculating works when you measure a values which be aimed at resistor.

Andrew Cowan

12-10-2008, 10:14

Which resistor value you use, depending what is your's voltage.

U=Voltage

I=Current

R=Resistor value (Ohms)

R=U/I

I=U/R

This calculating works when you measure a values which be aimed at resistor.

Make sure you incorperate the voltage drop from the LED if you do this. (2.2 or 2.1V)

Andrew

Protolisk

12-10-2008, 10:21

Which resistor value you use, depending what is your's voltage.

U=Voltage

I=Current

R=Resistor value (Ohms)

R=U/I

I=U/R

This calculating works when you measure a values which be aimed at resistor.

Thats what I'm trying to find out but I'm wondering if anyone would know around how much mA this would be. 20? 10? 5??

BeanieBots

12-10-2008, 10:26

6mcd is quite a lot so they can probably take quite a bit of current. Have a look in spec sheets for similar rated LEDs to get a feel for the current.

From about 60% max current upwards, there is not very much eye percievable increase in light output so it wouldn't be too difficult to work out the maximum current by experimentation/observation.

Start at about 20mA if you want to run them at full brightness.

If not, then 10mA will be a safe bet.

Protolisk

12-10-2008, 10:32

alright thanks

Dr_Acula

12-10-2008, 14:03

V=IR. 5V supply. 2.2V led. Leaves 2.8V left over. R=V/I which is 2.8/0.01 for 10mA. 280 ohms. But I still like 1k because even though it is a lot less current (2.8mA), modern leds are so bright isn't worth putting the extra current through. Plus at 2.8mA, batteries last longer. Plus you can drive many leds from a picaxe without overheating the chip. Plus you get less voltage ripple on the power supply when the leds go on and off. Try 220 ohms and try 1k and see if there is much of a brightness difference.

Protolisk

13-10-2008, 07:24

Well I've got a multimeter I dont really know how to use is there a way to find out the forward current with it.

BeanieBots

13-10-2008, 08:01

The actual current is not very critical as long as it does not exceed the maximum value. The LED will give off a good amount of light even at currents well below the maximum value. So, you really need to experiment.

To measure current, you need to put your multimeter (set to current) in series with your LED/resistor. Start with a resistor value of 1k.

First, just use a few different value resistors and your multimeter on their own. Say you have a 4.5v volt supply (3 X AA battery) and a 1k resistor. Your multimeter should show close to 4.5mA. From ohms law, V=IR, rearranged gives I=V/R. So, for every volt across 1k, you should get 1mA.

Now put the LED in series with the resistor and multimeter. If it is the right way around, it should light up. Some voltage will be dropped across the LED as well as the resistor, so the current will be less than with just the resistor. A typical LED volt drop is about 1.2V but this value depends on the LED colour, how much current is flowing and the type of LED.

Hopefully, you now have 4.5v into 1k - LED - multimeter.

The multimeter should show about 3.3mA.

Now try a few different value resistors but don't try anything less than about 200 ohms or you might damage the LED.

westaust55

13-10-2008, 12:00

Based on datasheets like the following (for a 2.1V yellow LED with 120 deg view angle), I suggest that your SMD type LED will output 6mCD at 20mA.

Mycroft2152

13-10-2008, 12:08

This subject can be "nesbitted" to death.

If you are using a 5 volt supply, start with a 1K resistor and see how it looks. Reduce the resistance until you get an intensity that you like.

Myc

Dr_Acula

13-10-2008, 15:04

Re "Well I've got a multimeter I dont really know how to use is there a way to find out the forward current with it"

Put a 1 ohm resistor in series with whatever resistor you are using to drive the led (1k or whatever). Every junk box needs a 1 ohm resistor. Get a few 5W or 10W ones - they are really handy for all sorts of things. Now put your multimeter on the milivolts range and measure the volts on the 1ohm resistor. milivolts = milliamps. Volts=amps.

It is a simple application of Ohms law. V=IR and if R=1, then V=I.

And the real reason this is a good idea: Well, if you put the multimeter on the current range, and you set it to milliamps, and then you accidentally put it across the power supply, you blow the fuse in the multimeter. Or zap the whole device. Been there *grin*. But multimeters are much more robust measuring volts. So if you use that 1 ohm resistor, and you put it on millivolts instead of volts, nothing bad happens.

Ok, maybe only true for cheapie multimeters, but still, if you can only afford a cheapie you don't want to zap it. And my (2nd) cheapie from when I was 12 is still going.

Protolisk

14-10-2008, 08:06

alright nice I'll try that.

Protolisk

15-10-2008, 08:09

Well I measured it with just the 1k resistor and got 2.85 mA and then measured it wwith the LED and got 1.07 so does that mean that it has a forward current of 1.78 mA?

BeanieBots

15-10-2008, 09:41

No, that means you have given it a forward current of 1.07mA.

What that tells me is that your supply voltage is just under 3v and your LED volt drop at about 2mA is about 1.7V but I've made a lot of assumptions about how you've wired things up, so pretty meaningless really.

As we are now on post 16 of "how to drive an LED", there must be some very basic misunderstanding going on here. Either we have totally failed to understand your question or you don't understand what "forward current" means, or possibly even both.

So, maybe it's time to re-word and re-ask your question.

Your original question was

"Just want to know around how much the forward current is? "

This has been explained over and over, including some suggested values both as numbers and also how to calculate it. So, what is it that you REALLY want to know?

Protolisk

17-10-2008, 08:33

What I really want to know is what is this number(I) in finding out what value resistor to use with an LED?

Resistor value=(3-2.2)/I

westaust55

17-10-2008, 09:22

Did you look at the attachments I provided at Post 11.

I = 20mA = 0.020 Amps (Maximum)

Dear oh dear. Roll on holiday.

No-one can give you a 'recommended' resistor value unless they have the product Data Sheet.

At last you now give a supply voltage - that helps!!

What YOU should be doing is searching through DATA SHEETS from places like Farnell for similar spec LEDs.

I don't understand how you have all the specs except Imax? Are these some cheap chod from Ebay?

Specs can vary, but a little one like that should be safe (fingers crossed) at 20mA. (Specific 'low current' LEDs are an exception). Many are fine at 30mA continuous.

But no-one can guarantee it. If it pops then tough and buy some with a Product Data Sheet (sorry, that's life).

And, if you are going to persue electronics, PLEASE learn how to use a multimeter. Ask your teacher/relatives/friends for help. This is a really basic piece of kit which is IMPORTANT.

FINALLY: we are assuming you will be running DC through this mystery LED. You can go higher when pulsing, but this varies with duty too. But lets leave that until you have a device with some proper Data...

For now, assume 20 to 30mA.

As for the R value ... you can do the calcs from your equation (?).

If not, Westaust will do it for you :)

westaust55

17-10-2008, 11:46

Dear oh dear. Roll on holiday.

:

:

For now, assume 20 to 30mA.

As for the R value ... you can do the calcs from your equation (?).

If not, Westaust will do it for you :)

Dippy, Are you suggesting some do not own a calculator :rolleyes:

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